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When we are making a buffer solution by solutions of a weak acid and its salt like $\ce{CH3COOH}$ and $\ce{CH3COONa}$, or by a weak base and its salt like $\ce{NH3}$ and $\ce{NH4Cl}$, we can use the Henderson equation to determine its $\mathrm{pH}$. But, when we are making a buffer solution by two salts of a polyprotic acid like $\ce{Na2HPO4}$ and $\ce{NaH2PO4}$, how can we calculate the resulting $\mathrm{pH}$? Can Henderson's equation still be used? Or, is there any other formula?

For example, the pH of a solution of $\ce{NaHCO3}$ is given by the formula $\frac{1}{2}(\mathrm{p}K_\mathrm{a1} + \mathrm{p}K_\mathrm{a2})$. But what would be the formula if the salt is from a triprotic acid? Again, what will be the formula if the solution is a mixture of salts?

I need a theoretical calculation of the $\mathrm{pH}$, and this is not for any laboratory type experiment.

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    $\begingroup$ You can use Henderson-Hasselbach. The equations will independently hold for both equilibria. You use use the total amount in solution to compute the actual amounts. $\endgroup$
    – Zhe
    Nov 21, 2016 at 18:25
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    $\begingroup$ A useful internet search for this type of problem is 'systematic calculation of pH'. These calculations can get messy pretty quickly, so take it slow. $\endgroup$
    – BiggChemT
    Nov 21, 2016 at 21:48
  • $\begingroup$ @Zhe How can I use the equation? $\endgroup$
    – S R Maiti
    Nov 23, 2016 at 9:02
  • $\begingroup$ chemistry.stackexchange.com/questions/8254/… $\endgroup$ Nov 25, 2016 at 6:05

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